The present invention relates to a method for automatic focusing, automatic astigmatic matching and automatic adjustment of an aberration corrector, or the like, of a scanning electron microscope, in particular, a scanning electron microscope (SEM) and a scanning transmission electron microscope (STEM).
In an application instrument of a charged particle beam, such as an electron microscope or an ion beam processing instrument or the like, an observed image or a specimen is processed by irradiation of a convergent charged particle beam onto a specimen. Resolution or processing accuracy of these charged particle beam instruments is determined by a size of the convergent charged particle beam (probe), and in principle, smaller size of the probe (probe diameter) is capable of enhancing resolution or processing accuracy. Recently, development of an aberration corrector for application instruments of the charged particle beam has been promoted, and practical application thereof has been progressed. In the aberration corrector, inverse aberration is given for a probe beam, by applying a rotationally asymmetric electric field or magnetic field to the probe beam, by using a multipole lens. In this way, various aberrations can be cancelled, such as spherical aberration, chromatic aberration and the like, which generate at an object lens or a deflector lens of a charged particle optical system.
In the charged particle optical system of a conventional application instrument of a charged particle beam, an axis-rotationally symmetric lens has been used, and thus, in principle, the probe diameter was able to be adjusted to the minimal value, by matching an axis of each of the lenses and an axis of aperture diaphragm, and by adjusting focus and an astigmatic point of the objective lens. In addition, in execution of focus adjustment and astigmatic correction, the adjustment was executed by obtaining a probe image under different focus conditions and by selecting the case with the highest sharpness while comparing image sharpness at least in two directions. On the other hand, in the aberration corrector, because generating aberration is cancelled by giving inverse aberration, correct measurement of kind of aberration (aberration components) and amount of each of the aberration components, contained in the probe beam, is required to remove the aberration. Increase in the aberration may be incurred adversely, and effect of the aberration correction cannot be obtained, in the case where estimation thereof and suitable adjustment of the aberration corrector are not executed.
Because the kind and amount of the aberration components are estimated, based on deviation of a cross-sectional shape of the probe beam from complete round, correct measurement of the cross-sectional shape of the probe beam is required in order to measure the aberration components. In U.S. Pat. No. 6,858,844 B2, a method for estimating the shape of the probe beam by using deconvolution has been disclosed. Explanation will be given below briefly on this method.
Specimen images are taken in a just-focus (a state where the beam is converged onto the specimen), in an under-focus (a state where the beam is converged at the backward of the specimen), and in an over-focus (a state where the beam is converged at the frontward of the specimen), and then each of the images is subjected to Fourier transformation. The Fourier transformation of under-focused image is divided with the Fourier transformation of in-focused image to obtain a quotient. The above process can be expressed by convolution integral (convolution) like the following equation (1), by using h(x, y) as a SEM image, f(x, y) as intensity information of secondary electrons, reflection electrons and the like, which generates from the specimen (including information on surface property or substances of the specimen), and g(x, y) as probe intensity information:h(x,y)=∫∫f(u,v)g(x−u,y−v)dudv  (Equation 1)Expression thereof in Fourier space gives:H(X,Y)=F(X,Y)G(X,Y)  (Equation 2)where the equation, is held, and F and G are also those obtained by Fourier transformation of corresponding amounts.
The equation (2) is held for any of the amount for the just-focused image and the under- or over-focused image (de-focused images), and H0=FG0, and H1=FG1 are held, when expressed by using the additional subscript character 0 and additional subscript character 1, respectively. Because the intensity information of probe in the under- or over-focused state cancels specimen information, it is divided to provide the equation (3).G1=Go(H1/Ho)  (Equation 3)
G1 can be determined by assuming ideal Gaussian distribution or the like as the probe intensity distribution G0, and the probe intensity g1 in the de-focused state can be determined by inverse Fourier transformation of G1.
Similarly, in U.S. Pat. No. 7,095,031 B2, and U.S. Pat. No. 7,060,986 B2, a method for estimating a probe shape, by using the above-explained FFT deconvolution, has been disclosed.